In order to implement an anisotropic high rate etching method, for example, for silicon, using an inductive plasma source, it is necessary in one method, described, for example, in German Published Patent Application No. 42 41 045, to perform efficient sidewall passivation in as short a time as possible, during so-called passivating steps, and furthermore to achieve as high a concentration as possible of silicon-etching fluorine radicals during so-called etching steps. In order to achieve an etching rate that is as high as possible, it is conventional to work with as high as possible a high-frequency power at the inductive plasma source, and thereby to couple in as high as possible plasma powers into the generated inductively coupled plasma.
However, there are limits to these high-frequency powers, which result, on the one hand, from load capacity of the electrical components of the plasma source, but, on the other hand, are also of a process technology nature. Thus, high-frequency powers of an inductive plasma source reinforce harmful electrical intervention by the source region in the inductively coupled plasma generated, which deteriorate the etching results on the substrate wafer.
Also, in etching processes according to the kind described in German Published Patent Application No. 42 41 045, stability problems appear in the coupling in of the plasma in the changeover phases between etching and passivating steps. This is based on the fact that, in response to high power to be coupled in, in the kwatt range, power reflection and overvoltage appearing during the changeover phases can have a destructive effect on the electrical circuit of the plasma source (coil, connected capacitors, generator output stage).
On this point, German Published Patent Application No. 199 00 179 describes an inductive plasma source, further refined compared to the one described in German Published Patent Application No. 42 41 045, which, with the aid of a loss-free symmetrical high-frequency supply of the coil of the inductive plasma source, is suitable for especially high plasma powers, and generates an inductive plasma which is particularly poor in electrical interference induced voltages. But for this source type, there exists a practicable power limit of about 3 kwatt to 5 kwatt, above which the required high-frequency components become extremely expensive, or, with respect to plasma stability, problems take the upper hand.
A possible approach towards attaining higher etching rates within a manageable power scope is to raise the efficiency of the plasma generation. In this connection, using magnetic fields to raise plasma efficiency is conventional.
By applying a magnetic field to a plasma, as is conventional, the electron paths in the plasma are bent, and because of that the residence time of the electrons in the plasma is increased, i.e. the time until they reach a wall which absorbs the electrons, so that each electron can interact more often with surrounding gas atoms until it leaves the effective plasma excitation region. Such impact interactions between electrons and gas molecules lead to the desired ionization or dissociation of the gas molecules along with the release of radicals needed for the etching process.
According to the related art, a so-called “multipole confinement” includes a metallic, nonferromagnetic wall having a plurality of permanent magnets of alternating polarity, which reflects electrons from the wall outfitted with these magnets by the action of magnetic fields. Thereby, a higher electron density can be generated within this “multipole confinement”. An analogous RIE (reactive ion etching) source is marketed, for example, by TEGAL Corporation, Petaluma Calif. 94955-6020, USA, as a so-called “HRe− Source”.
Other conventional plasma source types further make use of a magnetic field having a field direction parallel to a substrate electrode. Thus, by using a kind of Helmholtz coil pair directly at the-substrate electrode, a field distribution is generated that is as homogeneous as possible, which leads to increased length of the electron paths, and thereby to the generation of greater plasma densities. For the further homogenization of the effects, this horizontally oriented magnetic field as, for example, in the MRIE (magnetically enhanced reactive ion etching) equipment of Allied Materials, Inc., Santa Clara Calif. 95054-3299, USA, can further be rotated slowly in a planar manner.
In the case of so-called ECR (electron cyclotron resonance) sources, it is conventional that one should tune a longitudinal magnetic field in such a way that the circulation frequency of the electrons in this magnetic field, the so-called cyclotron frequency, is resonant with the frequency of the coupled-in microwave radiation, at least in a certain volume range of the etching reactor. Thus, an especially efficient plasma excitation by microwave irradiation can occur at a sufficiently free path length of the electrons, which opens up to such ECR sources the low-pressure range of process pressures lower than 1 μbar as a field of application. In this connection, the low pressure is a necessary condition for a sufficiently great path length of the electrons and for efficient plasma excitation. At higher pressures, ECR sources rapidly become inefficient, and go over to an unwanted thermal plasma generation. The advantage of the magnetic inclusion and the resonant excitation are thereby lost to a considerable extent.
It follows from the formula for the cyclotron frequency ω=eB/m that B=mω/e, i.e., at the usually irradiated microwave frequency of 2.45 GHz the magnetic field strength required for cyclotron resonance is 87.6 mTesla.
This application is not simply transferable to the case of high-frequency excitation in the MHz range, i.e., the case of typical frequencies for ICP (inductively coupled plasma) sources, since the free path lengths of the electrons, required for this, assume extremely low, impracticable pressures. After all, an inductive plasma source for high rate etching methods has to be configured for a relatively high pressure range of approximately 30 to 100 μbar.
The usual high frequency of 13.56 MHz, used for inductive plasma excitation using ICP sources, would further, in the case of cyclotron resonance, imply a resonance field strength of only 0.5 mT. However, such a low field hardly has any remaining guidance function for the electrons. For a sufficient guidance function, i.e., suppression of wall losses of the electrons in an extended plasma volume, field strengths of 10 mTesla or rather several times 10 mTesla to 100 mTesla are required.
Magnetic coils in an ECR-type configuration are also usually placed above, or at the same height as the plasma source, so as to generate the greatest field strength directly at the location of plasma generation, and so as to have the greatest possible influence on the plasma generating mechanism there. In the direction of the substrate to be etched, then, the magnetic field strength decreases rapidly because of the divergence of the magnetic field, so that the guidance function of the magnetic field is no longer sufficiently present in such an arrangement.
Varying plasma power coupled in an inductively coupled plasma having a high-frequency electromagnetic alternating field, adiabatically, between individual method steps, especially alternating etching and passivating steps, is described in German Published Patent Application No. 199 19 832. Such an adiabatic power transition, i.e., a gradual running-up or lowering of the coupled-in plasma power along with simultaneous continuous matching of the impedance of the ICP source to the respective plasma impedance as a function of the coupled-in plasma power, with the aid of an automatic matching network or an impedance transformer (“matchbox”), makes it possible to control the explained problems with regard to power reflection and voltage magnification in response to switching on and off plasma powers in the range of 1 kwatt to 5 kwatt. However, in this connection, a typical duration of transient effects is in the range of 0.1 sec to 2 sec. Therefore, power changes cannot be made by this approach.